At present there are available a wide range of process options in the extraction and recovery of metal values from various mineral species, including sulphide minerals. In particular, methods of oxidative hydrometallurgy are often used, one of these such methods being oxidative leaching processes.
Conventional or traditional oxidative leaching processes typically utilise severe physico-chemical conditions in order to achieve acceptable rates of oxidation and/or final recoveries of metal values. The severe physico-chemical conditions of the conventional technology often involve leaching with temperatures in excess of 150° C. and total pressures in excess of 2000 kPa. These extreme conditions, typified by the so-called Sherritt process, place certain physical requirements upon the reaction vessels or autoclaves, and the oxygen supply requirements. In addition, retention times in the leach are long in order to ensure adequate extraction of the appropriate metal values, often involving retention times of greater than two to two and a half hours.
Some work has been conducted in an effort to provide a method of processing a mineral species which utilises an oxidative leach under what may be termed “relatively mild” conditions of pressure and temperature, relative to conventional oxidative hydrometallurgy technologies. In addition, the leach utilised in these processing methods consumes less oxygen than conventional processes. One obvious advantage of the ability to conduct an oxidative leach under these “mild conditions” is the substantial avoidance of the cost required to provide autoclaves built to withstand conditions of high pressure and temperature. Australian Patent Applications 27182/92 and 48867/00 describe methods for the processing of mineral species utilising an oxidative leach under mild conditions of temperature and pressure. These mild conditions are described as temperatures of less than about 120° C. and oxygen pressures of less than about 1000 kPa. The content of both these prior patent applications is incorporated herein by reference.
The reactions involved in the oxidative leaching of sulphide minerals are typically exothermic, thereby generating heat which is required to be dissipated in some manner if the advantages of running the oxidative leach at mild conditions are to be realised. The methods described in Applications 27182/92 and 48867/00 utilise, in pilot plant form, a number of autoclaves arranged in series, in which slurry passes from one autoclave to the next, at the end of which the slurry, if it is at this point above 100° C., may be flashed to effect a level of cooling. The primary cooling is achieved by injection of a cooling liquid into each autoclave as required to achieve the necessary cooling.
The above processing options progressively dilute the slurry during oxidative leaching and require energy be consumed in pumping of the cooling liquid.
In addition to typical flashing methods, in which slurry is discharged to a flash tank at the end of the autoclave train, conventional methods may also utilise jacketed autoclaves about which cooling liquid is circulated, and/or the use of other forms of heat exchange, including the use of common heat sinks.
Heat exchange surfaces can typically foul, requiring cost and time intensive maintenance. The large extent of the heat transfer surfaces required often makes their use impractical.
The design utilised above does not readily lend itself to up-scaling to commercial plants, largely as a result of each of the inefficiencies noted above in relation to the cooling methods employed. One available solution is to run the oxidative leach at a higher temperature, for example 150° C. This would allow a reduction in the size of the reaction vessels in any commercial plant. However, this does not avoid the typical problems of the prior art processes utilising temperatures greater than 120° C., which is preferably to be avoided. In addition, the decrease in size of the reaction vessels obtained through an increase in the reaction temperature from 100° C. to 150° C. is typically only as great as 40%. It is envisaged by the Applicant that greater efficiencies can be obtained through utilising the process of the present invention.
The improved leaching process of the present invention has as one object thereof to substantially overcome, or at least provide a useful alternative to, the processes of the prior art described hereinabove.
The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.
Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.